1. Field of the Invention
The present invention relates to an image rejection mixer that acquires a third signal, which is a single sideband signal, by subjecting a first pair of complex signals, which are 90 degrees out of phase with each other, and a second pair of complex signals, which are 90 degrees out of phase with each other, to analog multiplication, and more particularly to an image rejection mixer that uses a 90-degree phase shifter, which doubles as a frequency divider, to acquire a pair of complex signals, which are 90 degrees out of phase with each other.
More specifically, the present invention relates to an image rejection mixer that suppresses an odd-order harmonic component contained in a pair of complex signals, and more particularly to an image rejection mixer that suppresses a harmonic component without impairing the phase orthogonality of a pair of complex signals.
2. Description of Related Art
A wireless LAN is highlighted as a system that saves the user the bother of making a hard-wired LAN connection. The use of a wireless LAN eliminates the need for most of hard-wired cable connections in an office or other work space. Therefore, a personal computer (PC) and other communication terminal can be relocated with relative ease. In recent years, the demand for a wireless LAN system has remarkably increased due, for instance, the availability of a high-speed, low-priced wireless LAN system. In addition, the introduction of a personal area network (PAN) for information communication has recently been studied with a view toward establishing a small-scale wireless network for a plurality of electronic devices around human beings. For example, various wireless communication systems and devices, which use a 2.4 GHz band, 5 GHz band, or other frequency band that does not require authorization from the supervisory authorities, are stipulated.
Recently, an ultra-wideband (UWB) communication system, which wirelessly convey information that is placed on an extremely feeble impulse train, is highlighted as a wireless communication system that provides short-distance, ultrahigh-speed information conveyance, and expectations are running high for the commercialization of such a system. As an access control method for ultra-wideband communication, a data transmission system for a packet structure containing a preamble is now defined, for instance, by the IEEE802.15.3 standard.
If a wireless network is established in an indoor work environment where many devices exist together, it is conceivable that communication stations may be scattered to establish a plurality of networks one above the other. When a wireless network based on a single channel is used, there is no workaround if the ongoing communication is interrupted by another system or if the communication quality is degraded, for instance, by interference. To avoid such a problem, a multichannel communication system is used. In the multichannel communication system, which is provided with a plurality of frequency channels, one frequency channel is used for operations. If the communication quality is degraded, for instance, by interference, the ongoing network operation is maintained by means of frequency hopping in order to permit coexistence with the other networks.
When a wireless network is established indoors, a multipath environment is formed so that the employed receiver receives a combination of a direct wave and a plurality of reflected/delayed waves. Because of such a multipath environment, delay distortion (or frequency-selective fading) occurs, thereby causing an error in communications. As a result, intersymbol interference arises out of delay distortion.
For example, a multicarrier transmission method may be used to solve the above delay distortion problem. When outgoing data is to be transmitted through the use of the multicarrier transmission method, it is distributed to a plurality of carriers that differ in frequency. The band of each carrier then turns out to be a narrowband so that the data is not likely to be affected by frequency-selective fading.
In OFDM (Orthogonal Frequency Division Multiplexing), which is a typical multicarrier transmission method, the frequencies of carriers are set so that the carriers within a symbol period are orthogonal with respect to each other. When information is to be conveyed, serially transmitted information is subjected to serial/parallel conversion at symbol intervals, which are slower than the information transmission rate. A plurality of resulting output data are assigned to the carriers. The carriers are individually subjected to amplitude modulation and phase modulation. The resulting carriers are then subjected to an inverse FFT. As a result, the carriers are converted to time axis signals while the frequency axis orthogonalities of the carriers are maintained. For reception, the above operation is reversed. More specifically, an FFT is performed to convert the time axis signal to a frequency axis signal. Each carrier is demodulated in a manner appropriate for the employed modulation method. Finally, parallel/serial conversion is effected to reproduce the original information that was transmitted with a serial signal.
For example, the OFDM modulation method is adopted by the IEEE802.11a/g as a wireless LAN standard. Further, the IEEE802.15.3 working group is now engaged in the standardization of a UWB communication method that employs the OFDM modulation method, in addition to a DS-UWB method, which provides the maximum diffusion velocity for a DS information signal, and an impulse-UWB method, which transmits/receives an information signal made of impulse signal trains that are timed at intervals as short as several hundred picoseconds or so. As regards the OFDM-UWB communication method, an OFDM modulation method for performing frequency hopping (FH) for a 3.1-4.8 GHz frequency band with three 528 MHz wide subbands and using an IFFT/FFT having 128-point frequency bands is being studied (refer to Nonpatent Document 1).
The use of a multiband generator is required for frequency hopping. A high-precision multiband generator can be formed if it comprises a plurality of oscillators for generating their respective frequency bands. However, the use of such a multiband generator causes problems in terms of circuit area and power consumption. Therefore, there is a technical demand that a single oscillator perform frequency division to generate a plurality of frequency bands.
For example, multiband generation can be achieved by repeatedly frequency-dividing a single frequency generated from an oscillator and mixing the resulting frequency-divided outputs (that is, by outputting the sum of the frequencies or the difference between the frequencies).
FIG. 15 illustrates a frequency synthesis block (3-band mode) for hopping for use with a multiband OFDM system. As indicated in the figure, the signal center frequency can be synthesized (by frequency addition/subtraction) from a single oscillator (e.g., TCXO (temperature-compensated crystal oscillator)) by means of frequency division and mixing. Further, synthesis can also be accomplished by means of 528 MHz frequency division, which is required as a sample clock.
In the example shown in FIG. 15, ⅛ frequency division is first performed to derive a frequency of 528 MHz from a frequency of 4224 MHz, which is generated from the oscillator. A frequency of 264 MHz is then obtained by performing ½ frequency division.
Next, each mixer obtains a frequency of 4488 MHz by adding a frequency of 4224 MHz to a frequency of 264 MHz. Further, each mixer obtains a frequency of 3960 MHz by adding a frequency of 4224 MHz to a frequency of 264 MHz. Furthermore, each mixer obtains a frequency of 3422 MHz by subtracting a frequency of 528 MHz and a frequency of 264 MHz from a frequency of 4224 MHz.
An image rejection mixer can be used as a device for frequency addition or subtraction, that is, for mixing. FIG. 16 schematically illustrates the configuration of an image rejection mixer. The basic principle of the image rejection mixer will now be described with reference to FIG. 16. The reference numerals S104a and S104b denote LO signals that are orthogonal with respect to each other. The reference numerals S102a and S102b denote IF signal that are orthogonal with respect to each other. The reference numeral S103c denotes an RF signal. The following equations represent the LO signals:S104a=cos(ωlo·t)  (1)S104b=sin(ωlo·t)  (2)
The following equations represent the IF signals:S102a=cos(ωif·t)  (3)S102b=−sin(ωif·t)  (4)
In a first multiplier 103, signals S104a and S102a are multiplied together. In a second multiplier, signals S104b and S102b are multiplied together. In an adder 103c, the product of the second multiplier is subtracted from the product of the first multiplier. As a result, the RF signal S103c is expressed by Equation 5 below:S103c=cos(ωlo·t)·cos(ωif·t)−(−sin(ωlo·t)·sin(ωif·t))  (5)
Synthesis can be achieved by performing frequency addition/subtraction in accordance with the trigonometric addition theorem. Therefore, Equation 6 below can be obtained by modifying Equation 5 above.S103c=cos └(ωlo−ωif)·t┘  (6)
Consequently, the RF signal of a frequency obtained by subtracting the IF signal frequency from the DO signal frequency is output. The frequency component of an image, which is obtained by adding the LO signal frequency to the IF signal frequency, is not generated. This is the reason why the mixer is called an image rejection mixer. LO signals orthogonal with respect to each other and IF signals orthogonal with respect to each other are required for the configuration of an image rejection mixer.
FIG. 17 shows an example in which a frequency divider is used as a 90-degree phase shifter for acquiring IF signals that are orthogonal with respect to each other. For present-day cellular phones, a frequency divider is generally used as a 90-degree phase shifter. When LO and IF signals are generated from a common oscillator by means of frequency division, an advantage is provided because the number of employed 90-degree phase shifters and frequency synthesizers can be reduced (refer, for instance, to Patent Documents 1 and 2).
A master-slave D-latch shown in FIG. 18 is used as a frequency divider circuit. The reference numeral S101 denotes a differential input. The reference numerals S101a and S101b denote differential outputs. Two different phase relationships may exist between the input and output depending on the D-latch initial status. However, the phase relationship between signals S101a and S101b remains unchanged without regard to the phase relationship between the input and output. FIG. 19 shows input/output waveforms prevailing in either of the above-mentioned relationships. As indicated in the figure, signals S101a and S101b are subjected to ½ frequency division with respect to signal S101. The output waveform is a rectangular wave. It is expressed as the sum of a fundamental wave and odd-order harmonic as indicated in Equations 7 and 8 below. At each frequency, signals S101a and S101b are 90 degrees out of phase with each other.
                                                                        S                ⁢                                                                  ⁢                101                ⁢                a                            =                            ⁢                                                ∑                                      n                    =                    1                                    ∞                                ⁢                                                                            sin                      ⁡                                              (                                                  n                          ·                                                      π                            2                                                                          )                                                              n                                    ·                                      cos                    ⁡                                          (                                                                        sin                          ⁡                                                      (                                                          n                              ·                                                              π                                2                                                                                      )                                                                          ·                        n                        ·                                                  ω                          if                                                ·                        t                                            )                                                                                                                                              =                            ⁢                              -                                                      ∑                                          n                      =                      1                                        ∞                                    ⁢                                                                                    sin                        ⁡                                                  (                                                      n                            ·                                                          π                              2                                                                                )                                                                    n                                        ·                                          sin                      ⁡                                              (                                                                                                            sin                              ⁡                                                              (                                                                  n                                  ·                                                                      π                                    2                                                                                                  )                                                                                      ·                            n                            ·                                                          ω                              if                                                        ·                            t                                                    +                                                      π                            2                                                                          )                                                                                                                                                    (        7        )                                          S          ⁢                                          ⁢          101          ⁢          b                =                  -                                    ∑                              n                =                1                            ∞                        ⁢                                                            sin                  ⁡                                      (                                          n                      ·                                              π                        2                                                              )                                                  n                            ·                              sin                ⁡                                  (                                                            sin                      ⁡                                              (                                                  n                          ·                                                      π                            2                                                                          )                                                              ·                    n                    ·                                          ω                      if                                        ·                    t                                    )                                                                                        (        8        )            
Determining the sum of sine waves causes no problem. However, when frequency division is repeated, spurious products, which are impurities made of an odd-order harmonic, are contained in the output. The following explanation deals with the spurious products that are contained in the output when the output of a 90-degree phase shifter is directly applied to the image rejection mixer as an IF signal.
The two signals that are 90 degrees out of phase with each other can be handled together when a complex signal is used. When signal S101a is assigned to the real part of the complex signal with signal S101b assigned to the imaginary part, Equation 9 below is obtained from Equations 7 and 8 above:
                                          S            ⁢                                                  ⁢            101            ⁢            a                    +                                    j              ·              S                        ⁢                                                  ⁢            101            ⁢            b                          =                  (                                    ∑                              n                =                1                            ∞                        ⁢                                                            sin                  ⁡                                      (                                          n                      ·                                              π                        2                                                              )                                                  n                            ·                              exp                ⁡                                  (                                                            -                      j                                        ·                                          sin                      ⁡                                              (                                                  n                          ·                                                      π                            2                                                                          )                                                              ·                    n                    ·                                          ω                      if                                        ·                    t                                    )                                                              )                                    (        9        )            
When Equation 9 above is expanded to a fifth-order harmonic for simplification, Equation 10 below is obtained:
                                          S            ⁢                                                  ⁢            101            ⁢            a                    +                                    j              ·              S                        ⁢                                                  ⁢            101            ⁢            b                          =                              exp            ⁡                          (                                                -                  j                                ·                                  ω                  if                                ·                t                            )                                -                                    1              3                        ⁢                          exp              ⁡                              (                                  j                  ·                  3                  ·                                      ω                    if                                    ·                  t                                )                                              +                                          ⁢                                          ⁢                                    1              5                        ⁢                          exp              ⁡                              (                                                      -                    j                                    ·                  5                  ·                                      ω                    if                                    ·                  t                                )                                                                        (        10        )            
Likewise, if it is assumed that the LO signals are fundamental waves only, Equation 11 below is obtained:S104a+j·S104b=exp(j·ωlo·t)  (11)
As the output of the image rejection mixer, only the real part of the product of Equations 10 and 11 above is expressed as signal S103c as indicated by Equation 12 below. Equation 12 can be simplified to obtain Equation 13.
                              S          ⁢                                          ⁢          103          ⁢          c                =                  Re          ⁡                      [                                          exp                ⁡                                  [                                      j                    ·                                          (                                                                        ω                          lo                                                -                                                  ω                          if                                                                    )                                        ·                    t                                    ]                                            -                                                          ⁢                                                          ⁢                                                1                  3                                ·                                  exp                  ⁡                                      [                                          j                      ·                                              (                                                                              ω                            lo                                                    +                                                      3                            ·                                                          ω                              if                                                                                                      )                                            ·                      t                                        ]                                                              +                                                          ⁢                                                          ⁢                                                1                  5                                ·                                  exp                  ⁡                                      [                                          j                      ·                                              (                                                                              ω                            lo                                                    -                                                      5                            ·                                                          ω                              if                                                                                                      )                                            ·                      t                                        ]                                                                        ]                                              (        12        )                                          S          ⁢                                          ⁢          103          ⁢          c                =                              cos            ⁢                          ⌊                                                (                                                            ω                      lo                                        -                                          ω                      if                                                        )                                ·                t                            ⌋                                -                                          ⁢                                          ⁢                                                    1                3                            ·              cos                        ⁢                          ⌊                                                (                                                            ω                      lo                                        +                                          3                      ·                                              ω                        if                                                                              )                                ·                t                            ⌋                                +                                          ⁢                                          ⁢                                                    1                5                            ·              cos                        ⁢                          ⌊                                                (                                                            ω                      lo                                        -                                          5                      ·                                              ω                        if                                                                              )                                ·                t                            ⌋                                                          (        13        )            
If, for instance, the LO signals are 1056 MHz and the IF signals are 264 MHz, the resulting RF signal output is as indicated in the table below:
TABLE 1RF frequency generation formulaω1o −ωifω1o + 3ωifω1o− 5ωifRF signal frequency [MHz]7921848264RF signal level11/31/5RF signal relative level [dB]0−9.5−14.0
It is well to remember that the discussion is simplified by dealing with the IF signal harmonic only. When the LO signal harmonic is also taken into consideration, it is obvious that spurious products arise in various combinations. In the above table, ωlo−5ωif is a negative frequency. However, cos(ωt)=cos(−ωt). As far as real number signals are concerned, there is no distinction between positive and negative frequencies. Therefore, the RF signal is a positive frequency.
As is obvious from Table 1, the output RF signal is not limited to 792 MHz. Table 1 indicates that frequencies of 1848 MHz and 264 MHz are generated as spurious products at relatively great relative levels of −9.5 dB and −14 dB, respectively. The relative levels are equal to the harmonic level with respect to the IF signal fundamental wave. Therefore, the IF signal harmonic needs to be reduced for the purpose of reducing the spurious products contained in the RF signal.
In wireless communications, the available frequency spectrum is generally limited by the Radio Law and the spurious products need to be suppressed. In a conventional example, a bandpass filter 107 is additionally positioned subsequently to the image rejection mixer as indicated in FIG. 17 or lowpass filters 105, 106 are positioned between a 90-degree phase shifter 101 and multipliers 103a, 103b as indicated in FIG. 20 (refer, for instance, to Patent Document 3).
However, the use of a bandpass filter causes a problem where the parts cost and size both increase. If the amount of harmonic suppression is increased during the use of lowpass filters, the circuit scale increases, thereby increasing the power consumption. If a resistor and capacitor are mounted in a semiconductor circuit, the resistance and capacitance values generally contain an individual difference of approximately 15%. However, if the two lowpass filters differ in the amplitude characteristic or phase characteristic, the IF signal orthogonality deteriorates, thereby degrading the image rejection performance.
[Patent Document 1]    Japanese Patent JP-A No. 4437/1998
[Patent Document 2]    Japanese Patent JP-B No. 29089/1999
[Patent Document 3]    Japanese Patent JP-A No. 171296/2002
[Nonpatent Document 1]    IEEE802.15.3a TI Document <URL:    http://grouper.ieee.org/groups/802/15/pub/2003/May03    filename: 03142r2P802-15_TI-CFP-Document.doc>